Power semiconductor devices are widely used to carry large currents, support high voltages and/or operate at high frequencies such as radio frequencies. A wide variety of power semiconductor devices are known in the art including, for example, High Electron Mobility Transistors (HEMT) and Metal Semiconductor Field Effect Transistors (MESFETs). Modern power semiconductor devices are generally fabricated from wide bandgap semiconductor materials. For example, HEMTs may be fabricated from silicon or, more recently, from gallium nitride (GaN) material systems on a silicon carbide (SiC) substrate. The power device may be marketed as a discrete power device or may be integrated in a Monolithic Microwave Integrated Circuit (MMIC). An overview of GaN-on-SiC HEMTs and MMICs may be found in the invited paper entitled “A Review of GaN on SiC High Electron-Mobility Power Transistors and MMICs” by Pengelly et al., IEEE Transactions on Microwave Theory and Techniques, Vol. 60, No. 6, June 2012, pp. 1764-1783.
Field effect transistors may be classified into depletion mode and enhancement mode types, corresponding to whether the transistor is in an ON-state or an OFF-state at zero gate voltage. In enhancement mode devices, the devices are OFF at zero gate voltage, whereas in depletion mode devices, the device is ON at zero gate voltage. HEMTs and MESFETs are typically depletion mode devices, in that they are conductive at zero gate bias due to the polarization-induced charge at a barrier/channel interface. Examples of such depletion mode devices include gallium arsenide (GaAs) MESFETs, GaAs pHEMTs and GaN HEMTs. Depletion mode GaN HEMTs that are marketed by Cree, Inc. are described in the following data sheets: “CMPA2735075D-75 W, 2.7-3.5 GHz, GaN MMIC, Power Amplifier” (Rev 1.1—April 2012); “CMPA0060025D-25 W, 20 MHz-6.0 GHz, GaN MMIC, Power Amplifier” (Rev 1.2—December 2014); “CMPA1D1E025F-25 W, 13.75-14.5 GHz, 40 V, Ku-Band GaN MMIC, Power Amplifier” (Rev 0.0—March 2015); “CMPA801B025D-25 W, 8.0-11.0 GHz, GaN MMIC, Power Amplifier” (Rev 1.0—June 2014); “CMPA2560025D-25 W, 2.5-6.0 GHz, GaN MMIC, Power Amplifier” (Rev 1.3—September 2012); “CMPA1D1E030D-30 W, 13.75-14.5 GHz, 40 V, GaN MMIC, Power Amplifier” (Rev 0.0—April 2015); and “CMPA5585025F-25 W, 5.5-8.5 GHz, GaN MMIC, Power Amplifier” (Rev 3.2—March 2015). Other depletion mode power devices may also be provided, such as depletion mode power Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) or Junction Field Effect Transistors (JFETs).